Field of the Invention
The present invention relates to a method of coating a fiber-reinforced plastic substrate, more particularly, to a method of applying a composite coating comprising a primer and a topcoat to the fiber-reinforced plastic substrate.
Brief Description of the Prior Art
Fiber-reinforced plastic is becoming an increasingly important material for the construction of automobiles and trucks. The materials most commonly used are resins such as polyesters which are reinforced with glass fibers. The mixture of resin and fibers is formed into a mass called bulk molding compound (BMC) or a sheet commonly called sheet molding compound (SMC). The SMC or BMC is compression molded to form the desired part.
Although the fiber-reinforced plastic parts have many advantages for automobile and truck usage, namely, they are strong, light in weight and do not rust, they also have some serious disadvantages associated with them. One disadvantage is that it is difficult to coat the fiber-reinforced plastic part and provide coatings with excellent appearance and good coating properties. The fiber-reinforced plastic parts have many surface imperfections, that is, porosity and microcracking, in the surface of the coating and surface depressions commonly called sink marks. It is very difficult to coat this type of substrate and achieve the same smooth glossy surface finishes as would be obtained with the painting of steel sheet. Even though the substrate is primed, many of the imperfections show through the primer and topcoat and are visible to the eye. Also, coating properties such as adhesion and stone chip resistance are often not as good when the coating is applied to a fiber-reinforced plastic as compared to application over steel.
To overcome some of these problems, the compression molders have been priming the fiber-reinforced plastic substrates with so-called "in-mold" coatings. In using these coatings, the mold containing the fiber-reinforced plastic part is partially opened during the molding cycle and the coating composition charged to the mold. The mold is closed and pressure applied to cause the coating to cover and to adhere to the substrate. Examples of "in-mold" coatings and their application methods are described in U.S. Pat. Nos. 4,189,517 and 4,086,578.
In-mold primers are difficult to work with requiring interruption of the molding cycle and charging a partially open mold with coating composition, a difficult procedure. Also, many of the in-mold primers are based on polyisocyanate resins which are toxic materials and are difficult to handle in a safe, healthful manner.
It would be easier to simply apply the primer directly to the fiber-reinforced plastic substrate at ambient conditions of pressure. Although there are commercially available compositions which can be applied in this manner, they also have shortcomings associated with them. One shortcoming is their sensitivity to overbake conditions. Usually, the coatings are cured at relatively low temperatures, that is, 225.degree.-325.degree. F. (107.degree.-163.degree. C.). The cured coating may then be exposed to higher temperatures which will adversely affect the properties of the primer such as its adhesion to the substrate. For example, the fiber-reinforced parts may be primed and cured at low temperature, i.e., 25.degree.-325.degree. F. (107.degree.-163.degree. C.) and then assembled to the automobile prior to electrodeposition. In this case, the primer must be able to withstand the higher curing temperatures for the electrodeposition coating, i.e., 40.degree.-390.degree. F. (171.degree.-199.degree. C.).
Therefore, it would be desirable to provide a primer which could be cured at a relatively low temperature, yet maintain its properties when subjected to higher temperatures.